This invention relates generally to gauges and, more specifically, to a component of a gauge that resists corrosion.
The bourdon tube, as invented by E. Bourdon in U.S. Pat. No. 9,163, is a tube having a flat cross section, e.g., an elliptical or rectangular cross section, which is bent in a coil form (e.g., spirally or helically wound) or a xe2x80x9cCxe2x80x9d shape (e.g., xc2xe of a circle). One end of the bourdon tube is stationary and has an open inlet which communicates with a source of pressure. The other end of the bourdon tube is sealed and allowed to move freely in proportion to the difference in pressure between the interior and exterior of the tube. As the pressure within the tube increases, the tube tends to straighten, resulting in a greater deflection. The amount of deflection of the free end of the tube is translated, mechanically or electrically, into a calibrated output indication of pressure.
Many factors must be considered in bourdon tube design to meet the requirements of the application. Factors that affect bourdon tube performance include pressure range, spring rate, corrosion-resistance to the external environment, repeatability, hysteresis, over-pressure, and ambient temperature effect. The bourdon tube shape, size, and material are selected for the particular application.
A particular concern is the corrosion resistance of the bourdon tube to internal fluid. The bourdon tube is typically made of a metallic material, for example, copper alloys, nickel alloys, or stainless steel.
One problem associated with copper alloys bourdon tubes is dealloying. Dealloying is a corrosion process in which the more active metal is selectively removed from an alloy, leaving behind a weak deposit of the more noble metal. Copper-zinc alloys containing more than 15% zinc are susceptible to a dealloying process called dezincification. In the dezincification of brass (a dark yellow alloy of copper and zinc), selective removal of zinc leaves a relatively porous and weak layer of copper and copper oxide. Corrosion of a similar nature continues beneath the primary corrosion layer, resulting in gradual replacement of sound brass with weak, porous copper. To address this problem, it is known to add inhibitors to admiralty brass in an attempt to reduce corrosion. Unfortunately, the addition of inhibitors in admiralty brass do not prevent dezincification in harmful soldering processes, such as at high temperature (e.g., 700xc2x0 F.) and processes using acidic flux.
Other copper alloys are also susceptible to dealloying. Copper-tin alloys are susceptible to a dealloying process known as desstannification. Green discloration results when a bourdon tube of the prior art is placed in contact with ISOVUE 370 solution (C17H22I3N3O8), often used in certain medical applications and manufactured by Bracco, Inc. of Princeton, N.J. For example, copper iodite, Cu(IO3)2, and/or copper iodite basic, Cu(OH)IO3, are formed as a reaction between the copper and ISOVUE 370 solution. One additional source of green discoloration is copper acetate, Cu(C2H3O2)2.H2O, which results from a reaction of copper and organic flux (i.e., acetic acid). In addition, a reaction between copper and standard flux (i.e., hydrobromic acid) can produce copper bromate, Cu(BrO3)2.6H2O.
Another problem associated with bourdon tubes containing copper is the formation of a green patina, Cu2CO3(OH), on copper and its alloys. This process is caused by the reaction between exposed metal and air containing moisture and carbon dioxide: 
The majority of green solid substance of copper alloy gauges is related to the formation of green patina. Copper oxide on the internal surfaces of the bourdon tube accelerates the formation of green patina. Gauges containing ISOVUE 370 are commonly used, for instance, to measure pressure of a fluid during angioplasty. The formation of green discolorant is undesirable during such use.
There is a need, therefore, for a component for a gauge which is resistant to corrosion. More specifically, there is a need for a component for a gauge which is resistant to dealloying and which is resistant to the formation of green discolorants, particularly in the presence of ISOVUE 370 solution which is typically used during angioplasty.
To meet these and other needs, and in view of its purposes, the present invention provides a component for a gauge, such as a pressure gauge, which is resistant to corrosion, including dealloying and the formation of green discolorants, such as copper iodite, copper bromate, and green patina. The invention provides a component for a gauge comprising a bourdon tube having a first end and a second end. A socket is joined to the first end of the bourdon tube and a closure member is joined to the second end of the bourdon tube. At least one of the bourdon tube and the closure member comprises a copper nickel alloy or the socket comprises a nickel silver alloy. The copper nickel alloy of the bourdon tube is preferably identical to, but may be different from, the copper nickel alloy of the closure member.
In one embodiment of the invention, the bourdon tube and/or closure member comprises 65-95%, preferably 70-90%, and most preferably about 70% copper and 5-35%, preferably 10-30%, and most preferably about 30% nickel. In another embodiment of the invention, the socket comprises copper, nickel, and zinc. Preferably, the socket comprises copper, nickel, zinc, manganese, and lead.
In yet another embodiment, the socket is joined to the first end of the bourdon tube by a low temperature tin-bismuth soldering material. In another embodiment, the closure member is joined to the second end of the bourdon tube by a low temperature tin-bismuth soldering material.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.